Method for producing a large-mass ohmic resistor for protecting eletronic assemblies from surges, and an electronic assembly

ABSTRACT

The invention relates to an electronic assembly, in particular for low power consumption electric switching devices such as low power contactors, time relays or the like. In order to provide protection against input current pulses, an ohmic resistor ( 6 ) is provided in the form of a resistive layer that is applied by pressing.

[0001] The present invention relates to a method for producing alarge-mass ohmic resistor for protecting electronic assemblies fromsurges and to such an electronic assembly having means for protectionfrom surge voltage or surge current pulses according to the definitionof the species in the independent claims.

[0002] In electronic assemblies, protection against high-energyovervoltage spikes is necessary to avoid destruction of the components.

[0003] IEC1000-4-5 for testing immunity to surge voltages containsinformation on the limiting values and waveforms for correspondingtests. Usual test levels are from 01.5 to 4 kV or higher, depending onthe required overvoltage category of the electronic assemblies. In theunloaded condition, the surge voltage pulse (surge load) has a doubleexponential waveform having a front time of 1.2 s and a time tohalf-value of 50 s. In this context, the front time is defined as therise time of a surge (voltage) pulse from 10% to 90% of its amplitudepeak value, whereas the time to half-value is defined as the time of thesurge (voltage) pulse from the maximum (100%) of the amplitude peakvalue to 50% thereof. In line-to-line testing, an impedance of 2 isspecified as the internal impedance of the surge generator.

[0004] Generally, it is not possible to design the components for suchan overvoltage. Therefore, it is usual to limit the voltage at theelectronic assembly using voltage-limiting components, for example, byvaristors or suppressor diodes, which are connected in parallel to thecomponent (FIG. 1). Due to the low internal impedance of the testgenerator, very large currents flow via the voltage-limiting protectionelement. In the process, a very large pulse energy must be absorbed.

[0005] To achieve a high protection level, it is therefore necessary touse a relatively large, voluminous varistor having a correspondingabsorbing capacity. Moreover, the clamping voltage, i.e., the voltagelimited by the protection element, increases as a function of thenon-linearity component of the protection element and of the currentpulse level.

[0006] As an example, in an application using a 275V varistor at a pulsevoltage of 4 kV with a phase angle of 90 to the line voltage, a maximumpulse current of about 1700A occurs, resulting in a maximum clampingvoltage of 900V. Since IEC1000-4-5 requires a sequence of twenty pulsesspaced sixty seconds apart, a varistor having a minimum diameter of 14mm is required for this load. In devices having a lower powerrequirement, it is therefore common practice to increase the inputimpedance of the electronic assembly by means of a series resistor (FIG.2) to reduce the pulse currents. In the example mentioned above, givenan input impedance of about 50, the pulse current is limited to amaximum value of 70A. On one hand, this results in a lower clampingvoltage of 750V maximum and, on the other hand, the varistor can bereduced to a disk diameter of 5 mm here. Because of this, it is alsopossible to use varistors in SMD technology (surface mounted device),which are currently only manufactured up to a maximum pulse loadcapacity of 1200A (single pulse).

[0007] In this context, it is a disadvantage that a resistor having avery high pulse immunity has to be used for the series resistor. In theexample mentioned above, the resistor must withstand a pulse power ofabout 240 kW. However, modern resistors in film technology areunsuitable for such a pulse load. Carbon composite resistors, which haveexcellent pulse immunity, are nowadays hardly produced anymore.Therefore, only wire-wound resistors are suitable as a series resistor.However, for the example mentioned above, a size having a power ratingof 4 Watts minimum is required, depending on the type of resistor.However, these resistors have a very voluminous design and are availableas through-hole mounted devices only. Moreover, the expenses are higherhere than when using a larger varistor for “hard clamping”, that is, fora suppressor circuit without additional series resistor.

[0008] Using SMD wire-wound resistors, which are currently manufacturedwith a maximum power rating of 2.5W, a surge load of 2 kV maximum can beachieved.

[0009] The object of the present invention is to provide a method forproducing a large-mass ohmic resistor and an electronic assemblyallowing for a smaller size for a specific surge load.

[0010] On the basis of assemblies of the type mentioned at the outset,this objective is achieved according to the present invention by thecharacterizing features of the independent claims while advantageousrefinements of the present invention can be gathered from the dependentclaims.

[0011] The method according to the present invention provides alarge-mass ohmic resistor which has a high pulse immunity and allows fora small size. To this end, a resistive film, in particular a carbonfilm, is deposited on a printed-circuit board between two terminal pads.This is advantageously done using the screen-printing technique.Preferably, the resistive film is made by multilayer printing. After theresistive film is deposited, it is baked and thus fixed on the substrate(printed-circuit board). Until now, carbon prints have only been used tosubstitute gold at the contact points or to make crossing conductivetracks (cross-overs).

[0012] In order to avoid a reduction in cross-section at the connectionor transition points between the resistive film and the terminal pad(s),the region of the printed circuit board between the terminal pads can beprovided with a filling layer in the region of the resistive film to bedeposited. In this manner, a flat substrate surface is provided for theresistive film. A stepped transition region in the edge region of theterminal pads is thus avoided. A further possibility of optimizing sucha transition region can be achieved by embedding the terminal pads in,for example, pre-milled recesses of the printed circuit board. In thismanner too, a flat substrate surface is achieved for the resistive film.

[0013] The electronic assembly according to the present inventionconstitutes the assembly for electrical devices having a powerconsumption preferably below 10 VA so that the power loss in the seriesresistor during nominal operation remains negligibly small (for 10VA,230V and Rv=50 , Pv<0.1W). The assembly is especially suitable forlow-power contactors, timing relays, and the like. In this context, theassembly is designed as a substrate element in the form of a printedcircuit board on which are located conductive tracks and terminal padsfor the connection of components, the printed circuit board beingpreferably provided with a resistive film on one side and, on the otherside, including components corresponding to the desired circuitfunction. The resistive film is preferably designed as a carbon film, asdescribed above. The components include semiconductor components and atleast one ohmic resistor acting as a series resistor. This seriesresistor is formed by the resistive film placed on the back of theprinted circuit board. This resistive film is plated through to thefront, and there it is connected into the electronic circuit as a seriesresistor for limiting pulse-shaped input currents. In this manner, theelectronic circuit is effectively protected from high-energy inputvoltage or input current spikes.

[0014] Further details and advantages of the present invention followfrom the following exemplary embodiment which will be explained on thebasis of Figures.

[0015]FIG. 1 shows a first possible input protection circuit of anelectronic assembly according to the prior art;

[0016]FIG. 2 shows a further possible input protection circuit of anelectronic assembly according to the prior art;

[0017]FIG. 3 shows an electronic assembly according to the presentinvention;

[0018]FIG. 4 is a detail view corresponding to FIG. 3;

[0019]FIG. 5 is a further detail view corresponding to FIG. 3;

[0020]FIGS. 6a-c are top views of the film resistor according to FIGS.3-5; and

[0021]FIG. 7 shows a further top view according to FIGS. 3-5.

[0022] According to FIG. 3, the electronic assembly includes a printedcircuit board 2, diverse components 4, such as semiconductors as well asactive and passive components. In order to protect components 4, inparticular the sensitive semiconductors, the electronic assemblyfeatures a series resistor 6 on the incoming side. Series resistor 6 ispreferably connected in series with at least one of the inputconnections (input terminals). According to the present invention, thisseries resistor 6 is designed in the form of an ohmic resistive film, inparticular, a carbon film. Preferably, the resistive film has amultilayer design and is possibly mixed with insulating pastes to attainthe desired resistance value. The resistive film is preferably designedas a carbon ink (e.g. 1-component carbon ink SD 2841 HAL or SD 2841HAL-BW from the Lackwerke Peters Company) and advantageously printed onprinted circuit board 2 using the screen-printing technique, andsubsequently baked. Through multiple printing, it is possible toincrease the layer thickness of resistor 4 and thus the pulse immunity.Advantageously, one side of the printed circuit board is only equippedwith components 4 while the other side of the printed circuit board isintended for printing with the resistive film. This allows for a smallsize of an assembly of that kind. For the interconnection of seriesresistor 6 (resistive film), terminal pads 8 of both sides of theprinted circuit board are plated through in known manner.

[0023] In order to avoid a reduction in cross-section at the connectionor transition points between the resistive film and terminal pad(s) 8,the region of printed circuit board 2 between terminal pads 8 can beprovided with a filling layer 10 in the region of the resistive film tobe deposited. In this manner, a flat substrate surface is provided forthe resistive film. A stepped transition region in the edge region ofterminal pads 8 is thus avoided.

[0024] A further possibility of optimizing such a transition region canbe achieved by embedding the terminal pads in, for example, pre-milledrecesses of the printed circuit board (not shown). In this manner too, aflat substrate surface is achieved for the resistive film.

[0025] In a of the carbon film of 0.002 m/mm², a layer thickness of 30 mand a resistor size of 7*20 mm, a resistance value of approximately 47is achieved. In this context, the active mass of resistor 6 is about 6.5mg, given a density of 1.55 g/cm³. In this context, given a permissibleshort-term temperature limit of approximately 300 C., a calculated,purely adiabatic absorption capacity of 1.6 Ws is achieved. Thiscorresponds to a current pulse of about 36A for the waveform mentionedat the outset, or to a surge load of about 2500V. However, empiricallydetermined values indicate a markedly higher pulse load capacity becauseof the excellent thermal coupling of the carbon film to printed circuitboard 2.

[0026] In practice, the weak point is the termination of the carbonfilm, i.e., the connection of the resistive film to the copper layer ofthe printed circuit board. As can be seen from the detail in FIG. 3, astep occurs at the transition of the printed carbon film from the copperlayer of terminal pad 8 to circuit board substrate 2. Due to this, theeffective resistive film is reduced at this location, and therefore thepulse load capacity is strongly reduced. Moreover, using the printingmethod, it is not possible to achieve optimum homogeneity at thislocation.

[0027] The reduction of the cross-section can be reduced by using aprinted circuit board 2 having as thin a copper plating as possible (forexample, 17 m instead of the 35 [ m] standard coating) and in that anintermediate layer, for example, a solder resist, which corresponds tothe copper thickness is printed on the intermediate surface.

[0028] Moreover, the transition area from the copper surface to theresistive film can be increased by a serrated (FIG. 6b) or waved (FIG.6c) shape of terminal pad 8.

[0029] A further way to relieve the termination is to taper theresistive film toward the middle of resistor 6. Through the tapering,the resistance value can be increased, resulting in a lower pulsecurrent, or the effective width of the termination can be increasedwhile maintaining same the resistance value.

[0030] The present invention is not limited to the specific embodimentsdescribed above but includes also all equally acting embodiments alongthe lines of the present invention. Moreover, all features shown in thedrawing belong to the present invention as well. In particular, thegeometry as shown. Thus, the application is not limited to the use of acarbon ink. In principle, all possible resistor pastes requiring a lowbaking or drying temperature (<200 C.) can be used. The nominal value ofthe resistor can be influenced within a wide range by the geometry ofthe print; however, the given example represents an excellent compromisebetween the pulse load capacity (here approximately 4 kV) and the powerloss of the resistor during nominal operation. Thus, the power loss ofan electronic assembly according to the present invention having a powerconsumption of 10VA is approximately 100 mW. If the nominal resistancevalue can be increased, for example, in the case of an electronicassembly having a lower power consumption, then the mass, i.e., thesurface of the carbon print can be reduced. In this context, the nominalresistance value can be achieved by reducing the conductance of thecarbon ink by mixing with insulating pastes. List of Reference Numerals2 printed circuit board 4 component 6 resistor 8 terminal pad 10 fillinglayer

What is claimed is:
 1. An electronic assembly, in particular forelectric switching devices having a low power consumption, such ascontactors, timing relays, or the like, comprising a printed circuitboard (2) including conductive tracks and terminal pads (8), the printedcircuit board (2) being equipped with semiconductor components (4) andat least one ohmic resistor (6) for limiting pulse-shaped inputcurrents, wherein the at least one ohmic resistor (6) is designed in theform of a printed resistive film.
 2. The electronic assembly as recitedin claim 1, wherein the resistive film is composed of a carbon ink, inparticular single-component carbon conductive ink.
 3. The electronicassembly as recited in one of the preceding claims, wherein a fillinglayer (10), in particular a solder resist layer, is arranged in theregion of the resistive film between the terminal pads (8) of theconductive tracks for the resistor (6) such that the printed circuitboard region for the resistive film, including the terminal pads (8),forms a transitionless flat surface.
 4. The electronic assembly asrecited in one of the preceding claims 1 through 3, wherein the terminalpads (8) for the connection of the resistive film are embedded in theprinted circuit board (2) such that the printed circuit board region forthe resistive film, including the terminal pads (8), forms atransitionless flat surface.
 5. The electronic assembly as recited inone of the preceding claims, wherein the printed circuit board pads (8)have a surface-enlarging, in particular a serrated or waved shape in theconnection region.
 6. The electronic assembly as recited in one of thepreceding claims, wherein the resistive film has a tapering shapebetween the terminal pads (8).
 7. The electronic assembly as recited inone of the preceding claims, wherein the printed circuit board (2) isequipped in such a manner that one side of the printed circuit board isprovided with discrete components, in particular only SMD components,and the other side of the printed circuit board is provided with theresistive film.
 8. A method for producing a large-mass ohmic seriesresistor having a high pulse immunity, wherein a resistive film isdeposited on a printed circuit board (2) having at least two terminalpads (8).
 9. The method as recited in the preceding claim, wherein theresistive film is deposited using the screen-printing technique.
 10. Themethod as recited in one of the two preceding claims, wherein theresistive film is a carbon layer.